Conductivity meters may be used to check if a liquid has reached a certain level inside a tube, a tank or the like. Metering is not continuous but at predetermined time intervals or occasions. One requirement for use of the method is of course that the liquid has sufficient conductivity and the method is often used for water or water based solutions. For this purpose one meters the current at a given voltage difference between two parts both of which are in the liquid if it has reached the level which is to be checked. One of the parts is often an electrode which is installed for this specific purpose, the other part may be another electrode, in many cases designed to surround the first electrode, or a part of the tube or the tank if these are made from electrically conductive goods. The method may be used both for flowing liquid and still liquid as in a holding tank.
When metering the liquid level inside a tank or a tube made from metal, a metering electrode or probe, which is electrically insulated from the surrounding goods, may be mounted into a tube socket so that the tip or outer end of the probe coincides with the plane of the tube wall or tank wall or slightly protrudes into the tube or the tank. When metering is to be done in heavily fouled liquid such as waste water it might be unsuitable to have parts protruding into the the liquid compartment as this may cause agglomeration of fouling material which stick to that part, which in turn may bring erronous metering results with it. One such application is described in the Swedish patent application no 9701153-0, which deals with evacuation of pumps at start-up. The level in a tube which is connected to the pump is then metered and the start-up procedure is selected due to the metering result. Small particles of fouling material may also adhere as a coating on that part of the surface of the probe which is exposed to the liquid and this may cause similar problems.